Thanks Brian. Here you go with answers.
As you are aware I use the @oopfan FITS Liberator balanced histogram with a bias frame for systematic checking off the offset setting before commencing an imaging run.
On 9/10 Oct I had excellent conditions - clear sky all night, excellent visibility, no wind and no moon. I could see many more lights over Manchester than usual. The offset (black level) setting of 50 gave an optimal bias histogram with my equipment and the 183C. I checked the camera settings file and 50 is correct. the temperature recorded in the file is 13.8.
On 14/15 Oct the conditions were poor. Although I started with clear sky (or so I thought) There was weather coming in from the south east around midnight. Last night I had 3 goes at offset settings (50, 100, 150) before I had an optimum histogram for a bias. I had expected 50 again as with the cap on the scope external conditions should not matter. The temperature recorded in the setting file is 8.
I think this is something I will have to monitor over a period of time unless you have any ideas.
M45 11 frames
I captured 20 frames - when I went back to the camera it had clouded over. When I came to calibrate and stack in Siril, only 11 frames would register. Hence the stack comprises the 11 frames with an integration time of 33 minutes.
The capture settings for M45 came out of a discussion with Gary Palmer at the international Astronomy Show over the weekend.
It gets better - look at this for the M31 image. Using Siril, the 20 frames were calibrated, registered, stacked + cropped, colour calibrated, green noise removal, colour saturation applied + a histogram transform (stretch) applied. Time taken around 15 minutes. The resultant TIFF was loaded into GIMP. Not far from being the finished article. A further 15 minutes in GIMP resulted in the image above.
This M31 and M45 are my first longer exposure images. What I have noticed in both cases was the resultant stack has needed less curves/levels manipulation. I am left with very little too do (I like that)! To me, this implies that the longer exposures are resulting in better quality data.
Look at the revision history for M45 at https://www.astrobin.com/370439/E/?nc=user
I have certainly had a quantum leap in my imaging results since experimenting with longer exposures. It will be interesting to see what I can capture with 180s frames in the Orion region. Of course, with longer exposures, the preparation needs to be meticulous - scope balance, polar alignment, mount align, focusing.
For anyone curious about the exposure debate try this simple experiment - M27: take 30x60s and 60x30s - same integration time, compare the results.
To date the tangible benefits I have seen from longer exposures (keeping the same integration time as previously) are, with larger frame cameras (5000x3000 @ RAW12 gives a 40Mb file), reduced disk space for capture and processing, faster calibration and stacking (less frames to process) and a better quality stack which results in reduced post processing.
As I start to use my Atik 314e and delve into narrowband I expect, in order to have decent results, to extend my expsoure times significantly.